An endoscope system having a solid-state imaging device consists mainly of an endoscope such as an electronic endoscope, a processor, a light source unit, and a monitor. In the endoscope system, the insertion unit of the endoscope is inserted into a body cavity, and illumination light emanating from the light source unit is irradiated to an object over a light guide lying through the endoscope. The solid-state imaging device incorporated in the distal part of the endoscope photoelectrically converts the light to produce a video signal. The processor processes the signal and displays an image on the monitor according to the signal.
Talking of the endoscope system, a field-sequential endoscope system like the one disclosed in, for example, Japanese Unexamined Patent Application Publication No. 1-221135 is known as a modality enabling observation under ordinary light by utilizing illumination light of wavelengths falling within the visible spectrum. In the endoscope system, as described in Japanese Unexamined Patent Application Publication No. 9-70384, an endoscope designed for fluorescence diagnosis is often employed in order to discover an early-stage carcinoma or the like. Specifically, excitation light is irradiated to a living tissue, and light stemming from fluorescence exhibited by the living tissue is observed in order to discover an early-stage carcinoma or the like.
An imaging device included in such a fluorescence diagnosis endoscope system is requested to offer so high sensitivity as to enable observation of feeble light stemming from fluorescence. For this reason, a pickup tube is often employed. Japanese Unexamined Patent Application Publication No. 5-252450 has disclosed a technology of controlling a drain voltage occurring due to overflow in a solid-state imaging device according to an output signal of the solid-state imaging device. The technology thus enables visualization of a region whose image cannot be corrected by controlling an amount of light using an iris diaphragm.